During papermaking process various chemicals are commonly added to enhance the physical properties of paper. Especially, there is a great variety of wet end additives designed for improving paper dry strength and/or wet strength. These additives typically have a given ionic charge to provide their molecules with sufficient affinity to be retained on cellulose fibers. Ionic polyacrylamides are well-known as such strength resins.
Ionic polyacrylamides are copolymers containing ionic monomers and acrylamide-containing monomers. The polyacrylamide backbone incorporates a small amount of an anionic or cationic monomer, e.g. diallyldimethyl ammonium chloride (DADMAC), rendering the polymer self-retaining on fibers.
These ionic polyacrylamide based additives are commonly modified to be more effective in improving the wet strength by use of crosslinking agents like glyoxal. GPAM is a reactive polymer that can covalently bind with cellulose upon dehydration. As an example, glyoxylated polyacrylamide (GPAM) is generally used in a variety of paper grades to enhance the dry and temporary wet strength. For example, GPAM increases the initial wet strength of many household tissues which come in contact with water in use. GPAM is also applied to increase the compression strength and the dimensional stability of many board-grade paper products.
Glyoxal and polyaldehyde compounds have been utilized as crosslinkers. However, glyoxal is very soluble in water and does not interact efficiently with other chemicals or compositions, particularly heterogeneous materials dispersed in small quantities in large volumes of water, e.g., such as gelatinized starch molecules or cellulosic fibers present in the wet-end of the paper making process. Thus, addition of glyoxal or other low molecular weight crosslinkers directly to the wet-end of the papermaking process has not been found to provide benefit to end product of the paper making process. Therefore ready-to-use compositions that contain the crosslinkers must be produced before addition to the wet end of a papermaking process.
The synthesis of GPAM was first reported in U.S. Pat. No. 3,556,932. The product was prepared by reacting glyoxal with a cationic polyacrylamide in slightly alkaline aqueous solution and stabilized under acidified condition.
The use of crosslinkers in such compositions is not straightforward, because problem arises regarding the stability and storage life of these compositions. Simple mixing of glyoxal with a polyacrylamide solution rapidly affords a gel. Under storage, the reaction between glyoxal and polyacrylamide continues, resulting in the increase of product viscosity over time and sometimes product gelling. Consequently, GPAM products are often produced at concentrations below 15% in order to extend shelf life.
Due to poor stability, regardless of pH, the molecular weights of current glyoxalated polyacrylamides continue to increase, even at room temperature, until the polymers gel. In most cases significant dilution to as low as 8.0% active solids concentration is needed. In addition to that, pH adjustment to 3.0-4.0 is required as well as storing at lower than room temperatures to ensure practical lengths of shelf lives. This increases costs especially during the summer or otherwise hot climate conditions, but most remarkable increase in costs becomes from the significant dilution of the composition, which affects the shipping and storing costs.
A variety of polymeric stabilizing agents have been discussed, but such agents increase the costs of the product and also increases the complexity of the composition. It is preferable that minimum amount of different chemicals would be put into the paper making process, because any additional chemical may cause problems in the complex paper making process.
U.S. Pat. No. 4,954,538 describes compositions comprising microparticles of a cross-linkable, glyoxalated (meth)acrylamide containing, polymeric material. The compositions are prepared using inverse microemulsion polymerization techniques and the products are said to be useful as wet- and dry-strength agents in paper production. It is indicated that commercial glyoxalated acrylamide polymers, supplied as 10% solid solutions, gel within about 8 days at room temperature.
U.S. Pat. No. 4,605,702 relates to the preparation of GPAM using a base polymer with a low molecular weight ranging from 500 Daltons to 6000 Daltons. The exemplified glyoxalated polyacrylamide products in U.S. Pat. No. 4,605,702 have a concentration of about 20% by weight cationic monomer, but were indicated as being used in much higher dosages to compare wet strength properties with exemplified products in US patent.
For the last decade, much research work has been carried out to reduce shipping and handling cost associated with GPAM products. U.S. Pat. No. 8,197,640 discloses a method for on-site GPAM production. Highly concentrated solutions of glyoxal and polyacrylamide are delivered to the papermaking mill separately. Afterwards at the paper mill, dilute GPAM products are prepared shortly before being applied to the paper machine.
The art continues to search for ways for producing glyoxalated polyacrylamide strengthening agents having good stability and at the same time lower the shipping costs.